net: bridge: provide shim definition for br_vlan_flags

br_vlan_replay() needs this, and we're preparing to move it to
br_switchdev.c, which will be compiled regardless of whether or not
CONFIG_BRIDGE_VLAN_FILTERING is enabled.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>